In general, the measurement of thermal conductivity and kinematic viscosity is essential to industrial process management for handling general fluids. It is well known to use a sensor which is capable of generating heat and measuring its own temperature, such that temperatures of the sensor itself and the temperature of the fluid to be monitored are measured. Thus, for example state of a fluid is determined on the basis of a characteristic kinematic viscosity value, defined as a function of a differential temperature of the sensor and fluid.
Such thermal methods for measuring the change in physical properties of a fluid are classified into two general methods, i.e. the steady-state and the unsteady-state methods. The unsteady-state method is based upon the temperature of the heating sensor, or a differential temperature between the heating sensor and the fluid, increasing with passage of time immediately after the sensor initiates the generation of heat. The so-called unsteady-state hot wire method, usually used to measure thermal conductivity of fluid, is one example of this unsteady-state method. On the other hand, the so-called steady-state method is based upon the phenomenon that, after a certain period of such heating of the sensor in the unsteady-state, a temperature of the heating sensor, or the differential temperature between the heating sensor and the fluid, becomes constant. It should be understood that, in response to any change in the physical properties of the fluid, such as viscosity, occurring in this steady state, the temperature of the heating sensor will change to a different level but will again stabilize at a constant temperature.
In the prior art, for example, the steady-state method is used to obtain values of various physical properties of a fluid on the basis of the respective temperatures, or to determine the state of a fluid from correlations between a change in temperature and a change in viscosity or other physical properties, as disclosed by the following documents:
A. Japanese Patent Publication No. 76702/1991 entitled "Method for Measurement of Change in Physical Properties of Fluid or Semisolid Substance" discloses a method for non-destructive measurement of a change in physical properties of a fluid comprising the steps of placing a metallic thin wire in the fluid, regulating the electric current applied to the metallic thin wire so that the difference between the temperature of the metallic thin wire and the temperature of the fluid may be maintained constant, and calculating the thermal conductivity of the fluid from the value of the electric current.
B. Japanese Patent Application Disclosure No. 185146/1987 entitled "Method for Fluid State Determination" discloses a method for determining the state of a fluid comprising the steps of placing a sensor in thermal contact with a fluid, measuring the temperature of the sensor itself and the temperature of the fluid, and intermittently or continuously measuring the difference between these temperatures.
For these techniques of the prior art, the temperature measured by the heating sensor is the most important value to be determined, and it is also essential to previously know the particular construction of the sensor, as well as the thermal characteristic of the sensor, or a particular correlation between the measured temperature and physical properties of the fluid.
The above-noted prior art "A" is the basic method for measuring the change in physical properties of a fluid utilizing the hot wire method and is exclusively used to determine a heat transfer coefficient that is different from the physical properties usually determined by that method.
The above-noted prior art "B" describes the determination of thermal conductivity and kinematic viscosity, which are the physical properties usually determined by that method. However, the specific method described therein is different from the method according to the present invention in that with the present invention fluid temperatures are measured inside and outside of a temperature boundary layer of laminar flow, formed around the heating sensor which is capable of generating heat and measuring its own temperature.